1. Field of the Invention
This invention relates to telephone supervisory signaling and, more particularly, to multiparty ringing control in multichannel subscriber loop carrier systems.
2. Description of the Prior Art
On long rural routes it is often economically attractive to provide a plurality of telephone channels on a single wire pair by using various carrier techniques. Such systems, known as subscriber loop carrier systems, must transmit all of the supervisory signaling required for telephone service as well as the two-way voice channel. It has been customary in prior systems, such as that disclosed in T. N. Rao et al U.S. Pat. No. 4,015,091, granted Mar. 29, 1977, to interrupt the carrier signal in order to transmit the supervisory information. For carrier systems of longer length, however, in which repeaters are necessary, it is usually necessary to maintain the carrier on at all times to control the automatic gain control circuits of the repeaters. Such a system is shown in the copending application of B. S. Bosik, Ser. No. 974,384, filed of even date herewith. In such cases, a single tone can be modulated on the transmitted carrier to encode the supervisory information. While such an arrangement is adequate for a single party service, it cannot be used for multiparty service where a plurality of different ringing states must be transmitted through the carrier system to the remote terminal.
Although many arrangements for encoding multiparty ringing states have been used in the past, these various systems have been incompatible and it was therefore necessary to select the proper signaling modem for a particular class of service and, moreover, to change these modems when a change in class of service occurred. This required the design, manufacture, and stocking of a number of different supervisory signaling modems, thereby increasing the cost and complexity of operating such carrier systems.